1. Field of the Disclosure
The present disclosure relates generally to determining geological properties of subsurface formations using Nuclear Magnetic Resonance (“NMR”) methods for logging wellbores, particularly the use of saturation pulse sequences in the presence of axial tool motion for dual wait time and T1 saturation recovery measurements.
2. Description of the Related Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction. The angle 0 through which the spins exactly on resonance are tipped is given by the equation:θ=γB1tp  (1),where γ is the gyromagnetic ratio, B1 is the effective field strength of the oscillating field and tp is the duration of the RF pulse.
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency ω0, given byωo=γB0  (2)where B0 is the static field intensity. At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time or T1. For hydrogen nuclei, γ/2π=4258 Hz/Gauss, so that a static field of 235 Gauss would produce a precession frequency of 1 MHz. T1 of fluid in pores is controlled totally by the molecular environment and is typically ten to one thousand milliseconds in rocks.
At the end of a θ=90° tipping pulse, spins on resonance are pointed in a common direction perpendicular to the static field, and they precess at the Larmor frequency. However, because of inhomogeneities in the static field due to the constraints on tool shape, imperfect instrumentation, or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. Hence, after a time long compared to the precession period, but shorter than T1, the spins will no longer be precessing in phase. This de-phasing occurs with a time constant that is commonly referred to as T2* if it is predominantly due to the static field inhomogeneity of the apparatus, and as T2 if it is due to properties of the material.
The receiving coil is designed so that a voltage is induced by the precessing spins. Only that component of the nuclear magnetization that is precessing in the plane perpendicular to the static field is sensed by the coil. After a 180° tipping pulse (or an “inversion pulse”), the spins on resonance are aligned opposite to the static field and the “precession” consists of a slow return along the static field axis to the equilibrium direction. Hence, a signal will be generated after a 90° tipping pulse, but not after a 180° tipping pulse in a generally uniform magnetic field.
Saturation in NMR means “destroying all magnetization”. Saturation is needed within Dual Wait Time (DTW) and T1 saturation recovery measurements. In the field of NMR the term T1 is called the longitudinal relaxation time while the term T2 is the transverse relaxation time. The DTW comprises two NMR echo trains, one starting with full magnetization after a long wait time TW (e.g. 6 s or more), the other starting after a shorter wait time of e.g. TW=1 s. If the NMR echoes of the two sequences are different then this is caused by formation components with T1>approx. 0.5 s. The DTW method, therefore, provides some information about T1. For the method to work it is important that for the sequence with TW=1 s, this sequence starts with zero magnetization, also called saturation. See, for example, U.S. Pat. No. 6,331,775 to Thern et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. An aperiodic saturation sequence (APS) works well where the logging tool is stationary (W. Dietrich et. al, Z. Anal. Chem. 279, 177-181 (1976)). If, on the other hand, the logging tool moves between the application of the saturation sequence and the following NMR echo sequence, then the saturated region is not coincident with the region where the NMR echoes come from. As a consequence, the saturation is not effective, and DTW measurements give wrong answers.
The use of wide-band saturation pulses that are relatively insensitive to lateral tool motion has been discussed in prior art. Not addressed in prior art is the issue of movement of the logging tool along the borehole axis. This can be a problem for a rate of penetration (ROP) greater than 20 m/h, depending upon the configuration of the logging tool. It should be noted that all the examples and discussions herein are specific to the NMR tool described in FIG. 2. With more elaborate wide-band saturation sequences, the problem can be overcome and effective saturation becomes possible even in excess of ROP=100 m/h.